Delta-sigma digital radiometric system

ABSTRACT

A passive radiometric system for thermally imaging objects in a scene. The system includes a digital square-law quantizer circuit including a plurality of comparators and a voltage divider network having a plurality of resistors. Each comparator receives a different reference signal generated by the voltage divider network and a common power signal from an antenna and outputs a high or low digital bit signal. The system also includes a delta-sigma circuit having a weighting table responsive to the digital bit signals from the comparators that converts the digital bit signals to a normalized bit word. The delta-sigma circuit also includes an accumulator that receives the digital bit words from the weighting table and provides an average of the digital bit words. The system also includes a digital-to-analog converter that converts the averaged bit words to an analog signal that is provided as a feedback signal to the quantizer circuit.

BACKGROUND

1. Field

This invention relates generally to a radiometric system for passivelydetecting and thermally imaging objects in a scene and, moreparticularly, to a radiometric system for passively detecting andthermally imaging objects in a scene, where the system employs a digitalsquare-law quantizer and delta-sigma feedback integrated on a commonchip using, for example, silicon-germanium (SiGe) and/or Sicomplementary metal oxide semiconductor (CMOS) fabrication technologies.

2. Discussion

Radiometric thermal imaging systems and cameras are known in the artthat passively detect and process signals in certain frequency bandsemitted from a particular scene. Some of these frequency bands, such asvarious RF bands in the 60-300 GHz frequency range, for example, 94 GHz(W-band), 140 GHz (D-band) and 220 GHz, are particularly useful becausethe emissions in those bands readily propagate through clouds, smoke,dust, fog, etc. Typically, the warmer or more emissive the object in thescene, the more energy the object emits at a particular frequency. Theradiometric system will detect the RF energy and convert it to arepresentative temperature value so that the different objects in thescene can be separately imaged and identified. In order to be suitableand effective, these radiometric systems generally require detectorsthat have a high sensitivity, low drift and can handle large backgroundtemperatures.

A typical radiometric camera that detects and images radiation in the RFbands often includes a focal plane array (FPA) that converts theradiation into an electric signal, where a lens focuses the radiationonto the array. The FPA typically includes a configuration of aplurality of receivers positioned in a two-dimensional plane, where eachof the receivers includes an antenna or signal horn having a pick-upprobe at the front end that converts the radiation to an electricalsignal that is amplified by a microwave monolithic integrated circuit(MMIC) low noise amplifier. A diode at the back end of the each receiverrectifies the amplified voltage signal to a DC signal, where the DCsignal amplitude is representative of the power level of the receivedsignal, which increases as the temperature of the object being imagedincreases, and where power and temperature are proportional to eachother. The DC voltage signal from each receiver is then digitized andconverted to an image, where higher voltages are displayed as whiterareas in the image representing warmer objects with higher radiometrictemperature.

One of the challenges associated with these types of radiometric camerasis providing a high enough thermal resolution between objects detectedin the scene. For example, in a typical scene, many of the objects willbe near room temperature, thus requiring significant temperatureresolution to distinguish those objects from each other. The sensitivityof the radiometer system depends on the receive system noise temperatureT, the captured radiometric bandwidth B and the integration time τ asfollows where it is assumed that the detector does not contributeinput-referred noise power, and that low frequency bias drift is notpresent.

${\delta \; T} = {\frac{T_{Sys}}{\sqrt{B\; \tau}}.}$

It is desirable that the operation of the radiometric system be asimmune from signal drift as possible as the temperature of the systemitself changes. In order to overcome a significant portion of thisanalog drift, it is known in the art to provide digital radiometricsystems where the received RF analog signal is converted to the digitaldomain as quickly as possible. However, there are advantages anddisadvantages to providing different levels and degrees of analogcircuitry and digital circuitry in these types of systems. For example,the antenna and amplifier capture signals over a wide range offrequencies B. Analog diodes can rectify an entire broad frequencyrange. However, these types of analog circuits that employ diodes andthe like often times have unacceptable signal drift and noise,especially at lower frequencies. Further, diode detectors typically donot have enough nonlinearity and are inefficient. Also, known digitalradiometric systems that may employ fast Fourier transform (FFT) or rootmean square (RMS) operations that convert digital frequency signals to arepresentative temperature typically require significant calculation athigh sampling and data rates.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a digital radiometric receiverarchitecture employing digital square-law detection and delta-sigmafeedback.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of the embodiments of the invention directed toa passive digital radiometric thermal imaging system is merely exemplaryin nature, and is in no way intended to limit the invention or itsapplications or uses. For example, although the discussion herein talksabout the radiometric system detecting RF and millimeter wave signals,the invention has application for a wider signal range, such as into theterahertz (THz) frequency bands.

The present invention proposes a digital radiometric system forthermally imaging a scene by passively detecting RF energy emitted fromthe scene and converting that RF energy to a thermal image. Throughadvances in silicon-germanium (SiGe) and Si complementary metal oxidesemiconductor (CMOS) fabrication technologies, effective digitalcircuits can be built and integrated on a single chip so that the RFsignals can be effectively converted to representative thermal signalswith high resolution and for higher frequency ranges. As will bediscussed, the radiometric system of the invention employs square-lawdetection to convert the RF signals to a representative digital signal,which employs a feedback signal from a delta-sigma circuit, where thesquare-law detector and the delta-sigma circuit are fabricated on acommon chip using SiGe and Si CMOS fabrication technologies. The digitalfront-end of the square-law detector minimizes drift in the receivesignal as a result of temperature changes of the system, and providesthe sensitivity required for high resolution. In one embodiment, thesquare-law detector provides high sensitivity, such as around 1 Kelvin,has low drift, such as under 100 PPM, and can handle large backgroundand amplifier noise temperatures, equivalent to as much as 3000 Kelvin.

FIG. 1 is a schematic diagram of a receiver architecture 10 for apassive digital radiometric system. The architecture 10 includes anantenna 12 that accepts the RF radiation from a scene at the desiredfrequency band of interest, where the antenna 12 is configured anddesigned for that particular frequency band. Signals received by theantenna 12 are provided to a receiver front-end 18 where they areamplified by a low noise amplifier (LNA) 14 and then down-converted toan intermediate frequency by a down-conversion circuit 16 to convert thehigh frequency received by the antenna 12 to a lower frequency moresuitable for operation by the digital circuitry of the architecture 10,as will be discussed below. For example, the antenna 12 may be designedto detect frequencies in the 90 GHz frequency band, and the circuit 16may convert that frequency down to about 10 GHz, where the lowerfrequency still provides high resolution and a low enough frequency foroperation of the digital circuitry. Employing the down-conversioncircuit 16 would typically be design specific in that requiring suchcircuitry would depend on the frequency band being detected and thespeed of the available digital electronics.

It is noted that the receiver architecture 10 and the antenna 12 may beone receiver architecture and antenna for one pixel of an array of manyreceiver architectures, where the antenna 12 may be part of a focalplane array (FPA). A typical FPA may include a configuration of aplurality of receivers positioned in a two-dimensional plane, where eachof the receivers includes an antenna or signal horn having a pick-upprobe. Alternately, the antenna 12 may be scanned across a scene throughsome scanning procedure, where the system only includes a singlereceiver architecture. It is also noted that for simplicity a unipolarcircuit architecture is shown, which processes only radio signals abovezero volts. One skilled in the art would have no difficulty constructinga bipolar equivalent handling both positive and negative thresholdlevels.

The down-converted RF signal from the receiver front-end 18 is sent to asquare-law quantizer circuit 20 that converts the analog signal to arepresentative digital signal. The square-law quantizer circuit 20includes a plurality of digital bit channels 22 each including acomparator 24 having a positive input and a negative input, where the RFsignal from the receiver front-end 18 is provided to the positive inputof each of the comparators 24, and where the output of each comparator24 represents a digital bit that is part of a digital word identifyingthe power of the incoming RF signal at any particular sample point intime. More particularly, if the power of the RF signal at the positiveinput of the comparator 24 is higher than a reference voltage at thenegative input of the comparator 24, then the output of the comparator24 goes high representing a digital one bit, where otherwise the outputof the comparator 24 is low representing a digital zero bit.

The square-law quantizer circuit 20 also includes a voltage dividernetwork 26 having a resistor ladder 28. Each of the resistors 28 isweighted with a different resistive value, where in this non-limitingexample, the lower resistor has the highest value and where theresistive value of the resistors 28 decreases towards the top of theresistor ladder 28. For example, in one non-limiting embodiment, thelowest of the resistors 28 may have an ohmic value of R, the secondlowest of the resistors may have an ohmic value of 0.414 R, the nextresistor 28 may have an ohmic value of 0.318 R, and the top resistor 28may have an ohmic value of 0.268 R. A current flow provided on afeedback line 30 discussed in detail below flowing through the voltagedivider network 26 provides a different voltage drop across each of theresistors. The negative input of each of the comparators 24 is coupledto a different location in the voltage divider network 26 so that adifferent potential is provided to the negative input of each of thecomparators 24.

In this design, the negative input of the comparators 24 is coupled tothe voltage divider network 26 so that the lower comparator 24 providesa logic one bit output for the lowest energy threshold level of the RFsignal, the next comparator 24 provides a logic one bit output for anext increase in power level of the RF signal, the next comparator 24provides a logic one bit output for a next increase in the power levelof the RF signal and for the higher power RF signals, all of thecomparators 24 will provide a logic one bit output. Although thisembodiment includes four of the bit channels 22, this is by way of anon-limiting example in that the number of the channels 22 would beapplication specific. For example, an increase in the number of channels22 may increase the bit resolution for a particular system, butincreases the electronics required to process the signals, and at somepoint there is a diminishing level of return for noise reduction andresolution.

The digital bits from the square-law quantizer circuit 20 are sent to aweighting and offset table 40 that performs mathematical operations onthe digital signal that then outputs the modified digital signal as adigital data stream on line 42. The weighting table 40 normalizes thedigital word based on the outputs from the comparators 24 and the valuesof the resistors 22 so that the digital signal is a value that fallswithin a particular range, such as zero to 1. The digital data stream onthe line 42 is sent to an accumulate and decimate circuit 44 thataccumulates the weighted data bits over time and then divides theaccumulated bits by the number of samples to provide an averaged digitalsignal representative of the temperature at any particular point intime, where the circuit 44 operates as a long term averaging filter.More particularly, the circuit 44 operates to filter out noise in thereceived RF signal at a particular point in time by averaging thetemperature signal over time to remove the noise. By averaging many ofthe digital signals over time, the resolution of the detected powersignal from the antenna 12 can be increased. In this way, for example,the square-law quantizer 20 processes billions of samples per second andthe circuit 44 outputs thousands of samples per second to provide therelatively noise free resolution. The normalized digital signal from thecircuit 44 provided on line 46 is the actual digital signal that is thenconverted to the thermal image, where that signal is provided on line 50for subsequent image processing in a manner that would be wellunderstood by those skilled in the art.

The weighting table 40 and the circuit 44 combine to operate as adelta-sigma circuit to provide a feedback signal on the line 30 to thevoltage divider network 26 so that the voltage potential across theresistors 28 changes depending on the power of the signal being receivedat any particular point in time. In order to provide the feedback signalon the line 30, each digital signal that is output from the circuit 44on the line 46 is converted to an analog signal by a digital-to-analogconverter (DAC) 48. If the particular RF signal provided by thefront-end 18 is at a higher end of the power range, then the feedbacksignal on the line 30 will change the voltage values applied to thenegative input of the comparators 24 so that the output of thecomparators 24 will not go high until a higher RF value is received,thus increasing the resolution of the system at the particular powerlevel being received at any point in time. Thus, by changing the voltagevalues provided to the comparators 24 through the feedback line 30, thebalance and ratio between the different comparators 24 is maintainedover a wider input power range.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion and from the accompanyingdrawings and claims that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

What is claimed is:
 1. A passive radiometric system for thermallyimaging objects in a scene, said system comprising: an antenna forreceiving signals from the scene and providing a power signal; a digitalsquare-law quantizer circuit including a plurality of comparators eachhaving first and second inputs, said square-law quantizer circuitfurther including a voltage divider network having a plurality ofweighted resistors and receiving an analog feedback signal, wherein eachcomparator receives a different voltage divided signal from the voltagedivider network at the first input and the power signal from the antennaat the second input and outputs a high or low digital bit signaldepending on whether the power signal is greater than or less than thevoltage divided signal; a delta-sigma circuit including a weightingtable responsive to the digital bit signals from the comparators, saidweighting table normalizing the digital bit signals to a digital bitword having a value within a normalized range, said delta-sigma circuitfurther including an accumulate and decimate circuit receiving thedigital bit words from the weighting table and accumulating the digitalwords over a predetermined sample time and dividing the accumulateddigital bit words by a number of sample periods to provide an average ofthe digital word that is a temperature representation of the powersignal; and a digital-to-analog converter (DAC) receiving the averageddigital bit word from the accumulate and decimate circuit and convertingthe averaged digital bit word to an analog signal provided as thefeedback signal to the voltage divider network.
 2. The system accordingto claim 1 wherein the digital square-law quantizer circuit and thedelta-sigma circuit are fabricated in silicon-germanium (SiGe) or Sicomplementary metal oxide semiconductor (CMOS) fabrication technologies.3. The system according to claim 2 wherein the square-law quantizercircuit and the delta-sigma circuit are integrated on a common chip. 4.The system according to claim 1 wherein the antenna is configured toreceive signals in the RF band.
 5. The system according to claim 4wherein the RF band is the W-band (94 GHz) or D-band (140 GHz).
 6. Thesystem according to claim 1 wherein the accumulate and decimate circuitis a long term averaging device for removing signal noise.
 7. The systemaccording to claim 1 wherein the plurality of comparators is fourcomparators and the plurality of resistors is four uniquely weightedresistors.
 8. The system according to claim 1 further comprising a lownoise amplifier (LNA) receiving the power signal from the antenna andamplifying the power signal.
 9. The system according to claim 8 furthercomprising a down-converter circuit receiving the power signal from theLNA and down-converting the power signal before it is applied to thedigital square-law quantizer circuit.
 10. A passive radiometric systemfor thermally imaging objects in a scene, said system comprising: anantenna for receiving signals from the scene and providing a powersignal; a digital square-law quantizer circuit responsive to the powersignal from the antenna and an analog feedback signal, said quantizercircuit converting the power signal to a digital signal; a delta-sigmacircuit responsive to the digital signals from the quantizer circuit,said delta-sigma circuit including a long term averaging device forremoving signal noise by providing an average of the digital signalsthat is a temperature representation of the power signal; and adigital-to-analog converter (DAC) receiving the averaged digital signalfrom the delta-sigma circuit and converting the averaged digital signalto an analog signal provided as the feedback signal to the quantizercircuit.
 11. The system according to claim 10 wherein the digitalsquare-law quantizer circuit includes a plurality of comparators eachhaving first and second inputs, said square-law quantizer circuitfurther including a voltage divider network having a plurality ofweighted resistors and receiving the analog feedback signal, whereineach comparator receives a different voltage divided signal from thevoltage divider network at the first input and the power signal from theantenna at the second input and outputs a high or low digital bit signaldepending on whether the power signal is greater than or less than thevoltage divided signal, where the combination of the digital bits fromthe comparators is the digital signal from the quantizer circuit. 12.The system according to claim 10 wherein the delta-sigma circuitincludes a weighting table responsive to the digital bit signals fromthe quantizer circuit, said weighting table normalizing the digital bitsignals to a digital bit word having a value within a normalized range,said delta-sigma circuit further including an accumulate and decimatecircuit receiving the digital bit words from the weighting table andaccumulating the digital words over a predetermined sample time anddividing the accumulated digital bit words by a number of sample periodsto provide the average of the digital signals.
 13. The system accordingto claim 10 wherein the digital square-law quantizer circuit and thedelta-sigma circuit are fabricated in silicon-germanium (SiGe) or Sicomplementary metal oxide semiconductor (CMOS) fabrication technologies.14. The system according to claim 13 wherein the square-law quantizercircuit and the delta-sigma circuit are integrated on a common chip. 15.The system according to claim 10 wherein the antenna is configured toreceive signals in the RF band.
 16. The system according to claim 15wherein the RF band is the W-band (94 GHz) or D-band (140 GHz).
 17. Amethod for thermally imaging objects in a scene, said method comprising:providing an antenna for receiving signals from the scene and acceptinga power signal; converting the power signal to a digital signal in adigital square-law quantizer circuit using an analog feedback signal;averaging the digital signal over time in a delta-sigma circuit togenerate an averaged digital signal that is a temperature representationof the power signal; and converting the averaged digital signal to theanalog feedback signal in a digital-to-analog converter (DAC) andproviding the feedback signal to the quantizer circuit.
 18. The methodaccording to claim 17 wherein converting the power signal to a digitalsignal in a digital square-law quantizer circuit includes providing aplurality of comparators each having first and second inputs, andproviding a voltage divider network having a plurality of weightedresistors that receive the analog feedback signal, wherein eachcomparator receives a different voltage divided signal from the voltagedivider network at the first input and the power signal from the antennaat the second input and outputs a high or low digital bit signaldepending on whether the power signal is greater than or less than thevoltage divided signal, where the combination of the digital bits fromthe comparators is the digital signal from the quantizer circuit. 19.The method according to claim 17 wherein averaging the digital signalover time in a delta-sigma circuit includes providing a weighting tableresponsive to the digital bit signals from the quantizer circuit, saidweighting table normalizing the digital bit signals to a digital bitword having a value within a normalized range, providing an accumulateand decimate circuit receiving the digital bit words from the weightingtable and accumulating the digital words over a predetermined sampletime and dividing the accumulated digital bit words by a number ofsample periods to provide the averaged digital signals.
 20. The methodaccording to claim 17 wherein the digital square-law quantizer circuitand the delta-sigma circuit are fabricated in silicon-germanium (SiGe)or Si complementary metal oxide semiconductor (CMOS) fabricationtechnologies on a common chip.